The success of Elachista fucicola as an epiphyte of Fucus vesiculosus is partly due to its ability to grow on most parts of the host thallus. Its ability to reach reproductive maturity quickly ensures its fitness, even on temporary structures such as receptacles. Epiphyte distribution may also vary with host morphology. Himanthalia elongata is a basiphyte for several tissue-specific epiphytes: Ectocarpus fasciculatus, Elachista scutulata, Herponema velutinum and Myriactula areschougii are exclusive, or almost so, to receptacles. On growing receptacles their specificity extends to conceptacles, which also provide refuges from grazing. Epiphyte abundance in summer is greatest at the upper and lower limits of Himanthalia distribution, suggesting that basiphytes living under suboptimal conditions are more prone to infestation by epiphytes. 5 10% of Himanthalia receptacles are thought to persist over winter and so to facilitate the transmission of epiphytes to the following year's crop of receptacles. The distribution of Ectocarpus fasciculatus on Laminaria digitata differs in pattern from that on Himanthalia. The two Ectocarpus populations also differ morphologically, but both have reproductive and developmental, characteristics that are in accord with the ecological cycles of their basiphytes. This characteristic is likely to be an important factor in determining the success of an epiphyte. Success may also be facilitated by epiphyte variability or by possession of a fixed but well-adapted phenotype. The very high incidence of brown algal epiphytes on growing Himanthalia receptacles does not reflect the composition of the surrounding algal vegetation. The possibility that some host-recognition mechanism operates cannot be discounted.